A linear accelerator, or linac, is a device used to accelerate electrons to high velocities. For most applications involving particle energies of greater than 1 MeV, radio frequency linear accelerators, or RF linear accelerators, are used. RF linear accelerators have many medical and industrial applications, including radiation therapy, medical and food product irradiation, polymer cross-linking, pest control, ion implantation in semiconductor production, and non-destructive testing.
A block diagram of a simplified RF linear accelerator system is shown in FIG. 1. The linear accelerator system 10 generally consists of an RF generator 11 to generate high power microwave pulses that set up electric fields in a linear accelerator waveguide 24, which are then used to accelerate electrons supplied by an electron gun 14 to produce a high-energy electron beam OUT. The accelerated electron beam strikes a target to produce the desired effect appropriate for the application. The RF power required to set up these accelerating fields is typically in the range of megawatts. Such power can only be produced in very short pulses, typically 1 μs to 20 μs. The RF generator 12 requires high voltage pulses and high amperage current to operate. The pulses to operate the RF generator 12 are created in a modulator 16. The modulator 16 requires high voltage DC, which is produced in a high voltage (HV) supply 18.
The function of the HV supply 18 is to produce the high voltages required for proper modulator operation. A typical modulator requires 10 to 12 kV at power levels of 10 to 100 kW. The HV supply 18 is generally regulated, filtered, and has some type of feedback of both the voltage and current.
The RF generator 11 uses an RF power device 12, such as a klystron, magnetron or other microwave source. The RF power device 12 requires an input control signal. In the case of a klystron, as illustrated in FIG. 1, this input control signal is provided by an RF driver 20. The output of the RF power device 12, is fed to a circulator 22, which buffers the microwave source from the accelerator waveguide 24. The accelerator waveguide 24 represents a highly dynamic load. At the beginning of a pulse, the waveguide 24 appears as a mismatch, and most of the RF power is reflected back towards the source 12. The reflected power is absorbed by a load 26 attached to one of the ports of the circulator 22.
The function of the modulator 16 is to provide high voltage pulses to the RF generator 11. The components of a typical modulator include a charging inductor, a pulse-forming network, a charging diode, a power switch tube, such as a thyratron, and a pulse transformer. In operation, the modulator 16 undergoes a charging cycle and a discharging cycle. On the charging cycle, the charging inductor and the capacitance of the pulse-forming network form a resonant circuit. This resonance causes the pulse-forming network to charge up to twice the voltage supplied by the HV supply 18. The charging diode keeps the pulse-forming network voltage at full voltage until the discharge cycle is initiated. The discharge cycle is initiated by conduction of the power switch. The components that operate in the discharge cycle are the power switch, the pulse forming network, the pulse transformer and RF power device 12 (klystron or magnetron). The discharge cycle results in a high voltage pulse appearing across the input of the RF power device 12.
Since the accelerator waveguide 24 operates at a very narrow range of frequencies, it is important that the RF generator 11 operate at the correct frequency. As the waveguide 24 heats up, the operating frequency shifts, requiring the RF generator 11 to track the change so as to maintain maximum output. Typically, there is an automatic frequency control circuit in the system 10 to automatically adjust the RF generator 11 to the correct frequency.
The preceding discussion outlines the general operation of a single beam linear accelerator. Multiple beam linear accelerator systems are also now in demand for many applications. Currently, multiple beam systems merely duplicate the components of the single beam system, and provide a common control and power platform. The klystrons and HV supplies are particularly expensive components when duplicated in a multiple beam system. Therefore, it is desirable to provide a multiple beam linear accelerator system that requires only one microwave source and ideally, only one HV supply.